Background: Although more than 1 million Veterans regularly use cannabis (i.e., have used cannabis within the past month), only limited research has investigated cannabis use among Veterans Health Administration (VHA) patients. Furthermore, existing health records do not ascertain use that is below the threshold of a cannabis use disorder, suggesting that a substantial portion of patients who regularly use cannabis are not documented or monitored by their VHA providers. This absence of data is concerning given the links between cannabis use and adverse physical, mental health, and social outcomes. Such consequences may be even more pronounced among certain population subgroups, including those with psychiatric illnesses, which are overrepresented in the VHA patient population. Despite potential harms, some patients who use cannabis report that it is beneficial for the management of chronic pain, posttraumatic stress disorder (PTSD), and other illnesses, and 23 States and the District of Columbia have legalized the use of cannabis for individuals with qualifying medical conditions. Research is needed to characterize and understand patterns of cannabis use and how they relate to health, functioning, and service utilization among VHA primary care patients. Objectives: 1) To characterize cannabis use among a representative sample of VHA primary care patients; 2) To examine the extent to which cannabis use is associated with psychoactive medication use (e.g., opiates and other psychotropics), substance use, substance use disorder symptoms, mental health symptoms (e.g., PTSD), pain, functioning, and treatment utilization among a cross-sectional sample of patients with regular cannabis and those with no past-year use; 3) To identify cannabis use and cannabis use disorder symptom trajectories at 6- and 12-months following an initial primary care visit; 4) To longitudinally estimate associations between cannabis trajectory groups and psychoactive medication use (e.g., opiates and other psychotropics), substance use, substance use disorder symptoms, mental health symptoms (e.g., PTSD), pain, functioning, and treatment utilization. A secondary aim is to compare use, trajectories, and outcomes between those with and without State-issued medical cannabis certification to examine how patients with medical cannabis certification differ from those without it in the nature and extent of cannabis use and problems. Methods: To achieve these objectives, the proposed project will screen Veterans receiving primary care at 3 VAMCs in Michigan. Through screening the project will identify a group of 500 patients with regular cannabis use, defined as at least monthly use during the past year, and an age- and sex-matched comparison group of 500 Veterans without past-year cannabis use. Both groups will complete an in-depth cross-sectional baseline interview. Following screening and baseline, the 500 patients with regular cannabis use will comprise a cohort that will be re-assessed at 6- and 12-months follow-up to identify cannabis use and cannabis use disorder symptom trajectories and related health, functioning, and service utilization outcomes.